Thread safety addresses the risk that when a program multitasks, one task may change the state of data being used by another task, causing that task’s understanding of the data to become incorrect.
In Ruby Concurrency Mechanisms, the Global Interpreter Lock provides implicit thread safety for traditional threads by ensuring only one thread executes Ruby code at a time. Many Ruby extension libraries are not thread-safe precisely because they expect to be protected by the GIL.
Ractors and Safety Through Isolation
Ractors achieve thread safety through a fundamentally different approach: the Actor Pattern enforces safety by making shared mutable state impossible. Instead of coordinating access through locks:
- Ractors have limited ability to access variables outside their scope
- Communication occurs only through specific, pre-defined message-passing mechanisms
- Most objects cannot be shared at all—only copied, moved, or marked immutable
This design prohibits the conditions that create race conditions, trading convenient shared state for guaranteed Parallelism without data corruption.
See Thread Contention is GVL Queuing in Ruby for how Ruby’s threading model manages Concurrency under GIL protection.